Magnetic bearings are used in many applications because, unlike contact-type bearings, there is no need for lubricants which, even in sealed systems, present contamination problems. For example, in FREON compressors, oil from mechanical bearings detrimentally affects the thermal properties of FREON and hence oil separator additives and liquid accumulators are required. In oxygen-based compressors, if oil becomes mixed with oxygen, the effect can be disastrous. Overall, magnetic bearings produce less heat, are more reliable and more maintainable than mechanical bearings and hence have a longer operating life. Magnetic bearings also offer improvements in efficiency, operating speed, operating temperature, shock tolerance, and mechanical configuration over mechanical bearings. Finally, the design of mechanical bearings for extended operations in zero gravity conditions is more complicated than the design of magnetic bearings for similar applications.
Conventional radial magnetic actuators apply forces to a rotating shaft to control motion perpendicular to the spin axis of the shaft and hence act in conjunction with a control system as a radial magnetic bearing. This is generally accomplished by surrounding the shaft with a radial actuator consisting of four quadrants each containing a two pole electromagnet. Each quadrant is excited by a single current control amplifier which produces a steady bias current and the control excitation. Alternatively, permanent magnets can be used in conjunction with electromagnetic coils. The permanent magnet surrounds the circumference of the shaft and is axially polarized. On each side of the permanent magnet, electromagnetic coils are wound on poles circumferentially located about the shaft. The magnetic flux generated by the electromagnetic coils and the permanent magnet is directed radially through one pair of air gaps between the shaft and the poles.
There are limitations, however, to both of these designs. For the former, the relationship between the force on the shaft and the control currents is non-linear which produces a variable gain. Therefore, the control loop required for force control requires fairly extensive design considerations. Furthermore, controlling the net force requires controlling at least two currents: at least one current for upward force and one current for downward force. Hence, separate power amplifiers for each quadrant are required for control. In addition, the power amplifiers must continuously provide the bias current even when no force is being produced on the rotating shaft. This requires high power capacity amplifiers and results in increased amplifier losses.
In the permanent magnet system, size is the primary limitation. For small systems used in outer space applications or even in some household appliances, there is simply no room for the substantial mechanical and electromechanical structure required in radial magnetic actuators incorporating permanent magnets. For example, for one application it was found a permanent magnet biased actuator had two to four times the length of an electromagnetic biased actuator. Permanent magnets are also expensive.